The ability of a brittle or thermoset epoxy resin to absorb energy without catastrophic failure can be increased through flexibilizing or toughening. Such flexibilizing and toughening may be accomplished by reacting or compounding the epoxy resin with an elastomer thereby enhancing the resin system's ability to resist mechanical and thermal stress. Such elastomers are known and include reactive liquid polymers such as dicarboxyl-terminated polymers as exemplified by U.S. Pat. No. 3,285,949, and diamine-terminated polymers as disclosed in U.S. Pat. No. 3,823,107. It is also known that liquid dicarboxyl-terminated polymers have the advantage of a material which is pourable and castable at room temperature and because of the reactive functional chain ends it can be further reacted at elevated temperatures by the addition of polyamines or diepoxies to form the liquid diamine or diepoxy terminated polymers. Such liquid elastomers have found a wide variety of utility, but are particularly useful as toughening agents in sealants, caulks, adhesive and potting epoxy resin systems. In fact, heretofore it was believed that difunctionality of these elastomers was essential to achieving desirable physical properties of the resins incorporating the elastomers. This thinking is typified by U.S. Pat. Nos. 3,285,949 and 3,823,107, and the article "The Chemistry of Rubber Toughened Epoxy Resins I." by A. R. Siebert and C. K. Riew. Improvements in such properties as the impact strength and crack resistance of resins incorporating carboxyl, amine, and epoxy-terminated polymers have been attributed to their difunctional character. The terminal carboxyl, amine, or epoxy functionality at or close to two was believed to be essential to achieving maximum toughness, and therefore polymers having lesser functionality were thought to be unsuitable.